This invention relates to integrated circuit input-output (I/O) circuitry, and more particularly, to adjustable differential integrated circuit input and output drivers for AC-coupled and DC-coupled communications links.
Input-output (I/O) circuitry is used on integrated circuits as an interface between the circuitry on the integrated circuit and external components. Input-output circuitry typically includes either single-ended or differential input drivers and single-ended or differential output drivers.
Single-ended drivers are used to handle single-ended signals—i.e., signals that are referenced to ground.
Differential drivers are used to handle differential signals, which are carried on pairs of conductors. Differential signals are referenced to each other, rather than a source of ground potential. One of the differential signals in each differential signal pair may be labeled “positive” and the other one of the differential signals in that pair may be labeled “negative.”
The average voltage of a differential signal is called its “common mode” voltage (VCM). To ensure proper operation of the differential input and output drivers in a high-speed link, the voltage swing of each of the differential signals (i.e., the voltage difference between the maximum and minimum of a differential signal) and the common-mode voltage of each signal should remain within certain prescribed ranges. Common-mode voltage bias circuitry is used to bias the drivers so that the differential signals remain within these ranges.
Differential communications links between integrated circuits may be AC-coupled or DC-coupled. The conductors in a DC-coupled communications link pass directly between an output driver on one circuit and a corresponding input driver on another circuit. In AC-coupled communications links, DC-blocking capacitors are used to block DC signals between the circuits, while allowing high-speed data signals to pass without interruption.
An advantage of AC-coupling in a link between two integrated circuits is that the capacitors effectively decouple the common-mode voltage level requirements of the output drivers of the first circuit from the common-mode voltage requirements of the input drivers of the second circuit. Each circuit can be biased as needed to ensure that the differential signal voltages remain within suitable limits. The DC-blocking capacitors also decouple the ground voltages of the two integrated circuits, which might not always be at the same level.
With a DC-coupled arrangement, only a single common-mode bias circuit is needed for each link and the DC-blocking capacitors are not required. Due to such considerations, DC-coupled arrangements are sometimes preferred by system designers.
With conventional DC-coupled arrangements, only a single common-mode bias circuit is typically used for each link. If bias circuits are used at both the output driver and the input driver of a link, these bias circuits can contend with each other. This may lead to undesirable power losses. Moreover, when the common-mode biases from each end of a link do not match, the common-mode voltage for the input and output drivers may settle to a level that is not optimal.
Conventional integrated circuits therefore generally contain differential I/O circuitry that is designed to support either AC-coupled communications or DC-coupled communications. If a system designer desires to use a DC-coupled link in a particular system, an integrated circuit with I/O circuitry designed for DC-coupled communications is used.
It would be desirable to be able to provide greater flexibility to system designers and to improve performance in high-speed differential signaling communications links.